Silicon carbide, a crystalline compound of silicon and carbide, has long been known for its hardness, its strength, and its excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties and maintains high strength at elevated temperatures. In recent years, the art of producing high-density silicon carbide bodies from silicon carbide powers has been developed. Methods include reaction bonding, chemical vapor deposition, hot pressing and pressureless sintering (initially forming the article and subsequently sintering). Examples of these methods are described in U.S. Pat. Nos. 3,853,566; 3,852,099; 3,954,483; and 3,960,577. The high-density silicon carbide bodies so produced are excellent engineering materials and find utility in fabrication of components for turbines, heat exchange units, pumps, and other equipment or tools that are exposed to severe wear, corrosion, and/or operation under high temperature conditions. The present invention relates to silicon carbide powders that are adapted to use in the various methods of producing a high-density silicon carbide body by sintering, and further to the use of the alpha crystalline form of silicon carbide in such processes.
The silicon carbide powder of the present invention may be admixed with various other materials that act as sintering aids; for example, materials containing carbon, beryllium, or boron, to form a sinterable mixture of the desired characteristics or composition. Such powder mixtures may be hot-pressed (simultaneous pressing and sintering) or may be cold-pressed with subsequent sintering to produce high-strength, high-density products. The product is substantially non-porous and eminently useful in engineering applications. If desired, the high-density, high-strength silicon carbide product may subsequently be machined, usually be diamond grinding, but also by electrochemical machining, ultrasonic machining, or by electrical discharge machining techniques, to provide tools or machine components requiring close tolerances.
One of the problems previously encountered in the utilization of silicon carbide mixtures is that, at the usual sintering temperatures, 1950.degree. to 2200.degree. C., beta phase silicon carbide converts to alpha. This results in the formation of large grains of alpha silicon carbide and a substantial weakening of the product.
Various methods of preventing or minimizing this phase change have been attempted, such as initial elimination of the alpha phase silicon carbide from the starting material, using a nitrogen atmosphere in the sintering operation, and sintering at lower temperatures. The present invention requires no precautions against phase change as alpha phase silicon carbide is utilized initially and higher sintering temperatures, up to about 2500.degree. C. are made possible.